Known milling cutter tools of this kind generally fall into two categories. In the first of these categories, the tool is provided with a single, peripherally disposed row of inserts, each insert being formed with a main cutting edge directed substantially parallel to the rotary axis of the cutting tool and designed to mill an upright wall of the recess or shoulder, and with a transversely directed secondary cutting edge designed to wipe or smooth a base wall of the recess. Such tools are, of course, limited as far as the depth of the recess which can be milled, this depth not being greater than the extent of the main cutting edge. In a second category of tools, the milling cutter is provided, in addition, to a lowermost row of cutting inserts of the kind just described, with one or more additional peripheral rows of inserts which are each provided with main cutting edges usually having positive axial rake angles. With such a tool, the extent of milling of the milled upright wall of the recess is, of course, increased. It will be understood that whilst the inserts comprising the lowermost row must be provided with the transversely-directed secondary cutting edges, the inserts comprising the second and any subsequent rows do not require the provision of such secondary cutting edges.
Furthermore, it is clearly desirable, from an economic point of view, that each constituent insert be provided with a maximum number of cutting edges which can be readily indexed into position as and when required. Most known milling cutter inserts used for the purpose indicated are either oblong-rectangular, and are therefore provided with two main cutting edges parallel to each other, or are triangular with three main cutting edges. In these known inserts, the main cutting edges may be associated with auxiliary or secondary cutting edges.
It has been proposed in EP 0 156 780 to use in a cylindrical milling cutter for the milling of recesses of the kind specified, wherein each insert is of a basic square shape having four indexable cutting edges, each main cutting edge being associated with a transversely-directed secondary cutting edge located on an extension of the insert which projects beyond the basic square shape thereof. The use of such inserts is, however, clearly limited. They can only serve in cylindrical milling cutting tools having a single row of inserts peripherally disposed on the cutter front face and therefore the depth of the recess formed by these inserts is limited to the extent of the cutting edge of each insert.
In an alternative proposal in EP 0 489 702 A2, there is disclosed an essentially square-shaped insert for a milling cutter having four main cutting edges, each main cutting edge having associated therewith a secondary cutting edge serving as a wiper, the main cutting edge extending slightly inwardly with respect to the associated secondary cutting edge at an angle, which is in the range of 0.5.degree. to 4.degree.. Such a cutting insert does not have projecting extensions which inherently restrict the depth of the recess. This insert, however, which is specified as being mounted at a negative radial rake angle, is subject to the essential problem present with such inserts, namely that the relief angle presented by the cutting edge at its leading end is very much greater than the relief angle presented by the cutting edge at its opposite trailing end. On the assumption that the relatively smaller relief angle presented by the insert at its trailing end is, in fact, the minimum relief angle which should be employed, it will be realized that the relief angle at the leading end is unduly large, thereby leading to an increased danger of weakening of the cutting edge at this point, and a consequent lowering of tool life.
This problem, which is inherent with conventional milling cutting inserts, has been recognized and discussed in the applicants' prior U.S. Pat. Nos. 5,071,292 and 5,078,550 and alternative proposals are made respectively in these prior patent specifications so as to overcome these problems or disadvantages. These proposals were, however, all made within the context of oblong-rectangular cutting inserts and their application to inserts of basically square shape wherein four alternative cutting edges were provided, was in no way envisaged.
The other disadvantage of the cutting insert, disclosed in EP 0 489 702, arises out of the configuration of the insert cutting edge, comprising as it does a main cutting edge portion and a secondary cutting edge portion inclined with respect thereto. The entire cutting edge is therefore non-rectilinear and seeing that both portions are in contact with the machined surface the degree of straightness and/or flatness of the machined surface which can be achieved is limited.
It is an object of the present invention to provide a new and improved exchangeable cutting insert for mounting, for example, in a milling cutter tool in which the above-referred-to disadvantages are substantially reduced or overcome.